The present disclosure is related to light emitting devices, and more specifically to a structure including an alternative cladding layer material used, for example, in devices emitting in the longer wavelengths, for example above 350 nm.
Semiconductor laser diodes (LDs) are compact, solid-state electronic devices capable of emitting light. A typical LD is comprised of a number of layers, including a lower and upper cladding layer, which provide optical confinement, and an active layer formed between these cladding layers. Typically, LDs are p-n junction devices, such that holes are injected from the p-region, and electrons are injected from the n-region. When electrons and holes combine in a depletion region between the p- and n-regions, a photon with energy equal to the difference between the electron and hole states is emitted. When a nearby photon with energy equal to the energy of the combining electron and hole (“recombination energy”) is emitted it can cause further electron-hole recombination by stimulated emission. This generates another photon of the same frequency, travelling in the same direction, with the same polarization and phase as the first photon. This means that stimulated emission causes gain in an optical wave (of the correct wavelength) in the active region, and the gain increases as the number of electrons and holes injected across the p-n junction increases.
Of particular interest in this disclosure is the wavelength (λ) of emission of a LD. This wavelength (λ) of emission is in part a function of the materials from which the active region is formed.
Production of LDs emitting light at less than 350 nanometers (nm) is well understood. As used herein, devices emitting light at less than 350 nanometers (nm) are referred to as shorter wavelength LDs. And conversely, as used herein, devices emitting light at and above 350 nm are referred to as longer wavelength LDs. While production of shorter wavelength LDs is generally well known today, there is still much effort being put into the development of functional, practical longer wavelength LDs. There are many important applications of longer wavelength LDs. Examples of such applications include optical recording and playback, xerography, optical communications, color displays (e.g., television and computer displays), etc.
Nitride-based materials have been used as an active region in LDs for commercial, longer wavelength devices. Gallium indium nitride (GaInN) is an example of such a material. However, although nitride ultraviolet (λ<380 nm), near-UV (λ≈405 nm), and violet-blue (405 nm≦λ≦470 nm) LDs have been demonstrated and produced commercially, their performance is not optimal and deteriorates with an increase in wavelength.
The sources of reduced LD performance at longer wavelengths are numerous. First, longer wavelengths imply an active GaInN region of higher indium content. When used as active regions, these alloys experience greater strain with respect to the gallium nitride (GaN) template they are typically formed upon. The higher strain is responsible for structural defects that affect the internal quantum efficiency. The greater strain is also responsible for a greater piezoelectric field across the quantum wells, which also reduces the radiative efficiency by separating the injected electrons and holes.
Furthermore, every material and material system has an inherent refractive index. For example, in the well known LD systems, a materials change in one layer results in a change in the refractive index of that layer. The refractive index difference between adjacent layers is critical to the mode shape, and hence efficiency, of the LD devices. A change in refractive index of only one layer of an adjacent pair changes that relative difference. As the actively layer indium content is increased the refractive index of that layer changes, and accordingly the relative index difference between the active layer and the adjacent cladding layer(s) change.
FIG. 1 is an illustration of device attributes of a generic nitride laser diode structure, and FIG. 2 shows a cross-section of such a generic nitride laser diode structure 20. Portion 12 of FIG. 1 shows a bandgap-energy representation, and portion 14 shows the corresponding refractive index profile associated with this structure.
FIG. 2 shows a cross section of such a device 20 including: sapphire (Al2O3) substrate 22, gallium nitride (GaN) template layer 23, lower aluminum gallium nitride (AlGaN) cladding layer 24, GaN or gallium indium nitride (GaInN) multiple quantum well (MQW) active layer 26, AlGaN electron blocking layer (EBL) 28, p-type GaN waveguide 30, AlGaN upper cladding layer 32, and GaN ohmic contact 34. Alternatively, substrate 22 may be comprised of GaN (not shown) in which case no additional GaN buffer layer need by used.
An optimized LD structure achieves both strong carrier confinement and optical confinement. The carrier confinement is realized by including high-bandgap alloys in the active region heterostructure (a structure comprised of at least two layers or regions of dissimilar crystalline semiconductors), specifically in the cladding layers surrounding the quantum well active layer. A cladding layer having a low refractive index produces strong optical confinement. Thus, as the active layer allow changes, compensation must be made in the cladding layers to maintain carrier confinement and optical confinement. One known approach to maintaining carrier and optical confinement is to substitute indium in the cladding layers for aluminum as the indium content in the active layer increases.
FIG. 3 shows the dispersion for GaN, and families of GaInN and AlGaN alloys, for example as used in the cladding layer of a LD heterostructure. For a typical range of alloys available to form a laser heterostructures at the longer wavelengths of 405 and 500 nm, the corresponding refractive index band is highlighted by the labeled vertical bars. That is, the bars indicate the range of alloys and refractive indices available to form conventional 405 nm and 500 nm nitride laser diodes, respectively. At longer wavelengths, the index range is smaller, unfortunately yielding weaker transverse optical mode confinement.
The poor optical confinement of a known 500 nm GaInN laser structure is illustrated in FIG. 4, which shows the aggregate maximum optical confinement factor (Γ, defined as the spatial overlap between the quantum well gain and the normalized optical mode) values for transverse waveguiding simulations of two laser structures, 405 nm emission and 500 nm emission, respectively. The structure of each of the devices is summarized in Table 1.
TABLE 1405 nm500 nmcladdingAl0.07Ga0.93NGa0.90In0.10NSCH (opt. Γ)Ga0.90In0.10NGa0.89In0.11NBarriers (10 nm)Ga0.98In0.02NGa0.88In0.12NQWs (3 nm)Ga0.90In0.10NGa0.73In0.27NEBL (15 nm)Al0.20Ga0.80NAl0.05Ga0.95N
Due in part to the substitution of In for Al in the cladding layers, the 500 nm LD structure in this example is designed to have similar strain as a conventional 405 nm LD, as well as similar bandgap energy differences for adequate carrier confinement. More specifically, the cladding layer is assumed to be semi-infinite Ga0.90In0.10N, the barriers are 10 nm thick Ga0.88In0.12N, the quantum wells (QWs) are 3 nm thick Ga0.73In0.27N, and the electron blocking layer (EBL) is 15 nm thick Ga0.95In0.05N. For this structure, the optical confinement factor was calculated for structures with different numbers of QWs (N=1, 2, 3, or 4). For each case, the Ga0.89In0.11N separate-confinement heterostructure (SCH) thickness was adjusted for maximum optical confinement factor Γ.
Note from FIG. 4 that the Γ values for the 500 nm laser are roughly half that of the 405 nm laser. High Γ represents desirable device performance, and the indicated values for the 500 nm device are below practicable values. This dictates that a larger number of QWs would be required to provide sufficient modal gain; this would also translate to a higher threshold current. Neither of these modifications are desirable.
Therefore, the range of alloy compositions available to form such heterostructures is limited. The three interrelated challenges which have heretofore limited production of practical longer wavelength LDs are: (1) the smaller refractive-index differences (i.e., lower dispersion) of GaInN alloys (for the active region) at longer wavelengths; (2) the longer wavelength itself (since the mode size scales with wavelength); and (3) the strain limitations that may preclude using aluminum gallium nitride (AlGaN) cladding layers (which are tensile-strained and prone to cracking).
Accordingly, described herein is an alternative nitride laser structure providing acceptable carrier and optical confinement for longer wavelength operation. Investigations into alternative upper cladding layers has led to the realization that such alternative cladding layers may have applicability not only in the longer wavelength devices, but in many other devices such as those emitting in the violet-blue region.